(1) Field of the Invention
The present invention relates to a light transmitting fiber comprising a novel cladding material.
(2) Description of the Related Art
Generally, in a step index type light transmitting fiber, the periphery of a core having a high refractive index is covered with a cladding having a low refractive index so that the entire internal total reflection is utilized, and the larger the difference of the refractive index between the core and cladding, the better the light transmitting property.
Multi-component glass, quartz glass or the like have been mainly used as the base of the core, and light transmitting fibers formed by covering this core with a cladding material composed of a glass or synthetic resin having a refractive index different from that of the core have been developed. However, since glass has a poor flexibility and the fibers are easily broken, a light transmitting fiber comprising a core of a synthetic fiber has been proposed. For example, a light transmitting fiber comprising a core composed of polystyrene and a cladding composed of an acrylic resin such as polymethyl methacrylate, and a light transmitting fiber comprising a core composed of polymethyl methacrylate and a cladding composed of a fluorine-containing polymer are now commercially available.
Of these commercially available light transmitting fibers, a light transmitting fiber comprising a core composed of polymethyl methacrylate has an excellent light-transmitting property and has a superior flexibility and heat resistance to a light transmitting fiber comprising a core composed of polystyrene. Therefore, this light transmitting fiber is used for short-distance light communication.
However, the refractive index of polymethyl methacrylate is 1.49, at the lowest level among generalpurpose resins, and therefore, if polymethyl methacrylate is used as the core, the number of polymers that can be used as the cladding is limited.
For example, Japanese Examined Patent Publications No. 43-8978, No. 54,24302, No. 56-49326, No. 56-8321, and No. 56-8322 disclose light transmitting fibers comprising a core of a polyalkyl methacrylate clad with a polyfluoroalkyl methacrylate. Japanese Examined Patent Publication No. 53-42260 discloses a light transmitting fiber comprising a vinylidene fluoride/tetrafluoroethylene copolymer as the cladding. The light-transmitting property of the light transmitting fiber is significantly influenced by the light absorbing and scattering properties of the core and cladding and the light reflectance in the boundary interface between the core and cladding.
A fluoroalkyl methacrylate polymer proposed as the cladding material exhibits a very small absorption and scattering and the light transmission loss caused by these two factors can be reduced, but this polymer has a poor heat resistance. If the heat resistance of the cladding is poor, although the core has a substantially sufficient heat resistance, the optical fiber as a whole has a poor heat resistance. Accordingly, the fluoroalkyl methacrylate polymer heretofore proposed as the cladding material has an insufficient heat resistance. Of the fluoroalkyl methacrylate polymers, a polymer of 2,2,2-trifluoroethyl methacrylate has a higher heat resistance, but the glass transition temperature of this polymer is 83.degree. C. and that of a polymer of 2,2,2,2',2',2'-hexafluoroisopropyl methacrylate is 95.degree. C. On the other hand, the glass transition temperature of polymethyl methacrylate is 100.degree. C., and a cladding material having a glass transition temperature higher than that of polymethyl methacrylate has not been developed.
The density of double bond electrons having a radical polymerizability in the fluoroalkyl methacrylate polymer is lower than in the corresponding alkyl methacrylate polymer, and the molecular structure readily undergoes radical depolymerization. Accordingly, the characteristics given to the fluoroalkyl methacrylate polymer at the time of production thereof are degraded, as the processing involves repeated shaping under heating. Also, the polymerization degree is reduced and the characteristics are drastically degraded by the plasticizing effect of the monomer formed by depolymerization.
These fluorine-containing polymers have an optical characteristic such as a low refractive index, as pointed out hereinbefore. Furthermore, the molecular structure of these polymers gives surface characteristics such as water-repellant and oil-repellant properties, as prominently observed in polytetrafluoroethylene. Fluorine-containing polymers now widely used as industrial material are characterized by such surface properties whereby they cannot be wetted by either water or oil. A fluoroalkyl methacrylate polymer has water-repellant and oil-repellant properties as well as these fluorinecontaining polymers but this polymer has extremely poor adhesion to the core, which is a very important property in the light transmitting fiber. Namely, the adhesion to not only polymethyl methacrylate as the core but also an acryl-modified resin having excellent heat resistance or an optical glass of the quartz or multi-component type is poor. In addition, the cladding of this polymer is easily separated from the core when the optical fiber is bent or drawn. Therefore, the fluoroalkyl methacrylate polymer has good optical characteristics but poor mechanical characteristics and heat resistance, so that the polymer cannot be put into practical use.
A vinylidene fluoride/tetrafluoroethylene copolymer such as a copolymer disclosed in Japanese Examined Patent Publication No. 53-42260 has excellent resistance against thermal decomposition and also excellent adhesion to polymethyl methacrylate as the core. Howeven, this polymer is crystalline and opaque, and accordingly, the transmission loss by scattering is very large. Therefore, if the polymer is held in an atmosphere maintained at 80.degree. to 90.degree. C., crystallization is advanced and the light transmission loss is greatly increased by scattering caused by the growth of the crystal.